Method of treating converter gas

ABSTRACT

Sodium carbonate is added to water used to scrub converter gases so that calcium-oxide particles in the gas react with the aqueous sodium carbonate and a thin inactive film of calcium carbonate is formed on the calcium-oxide particles, using carbon dioxide in the gas for the reaction. The coated particles are then separated from the wash water by settling and sedimentation and the wash water is recycled with the addition of more sodium carbonate and water.

FIELD OF THE INVENTION

The present invention relates to a method of removing particles fromgas. More particularly this invention concerns a system for scrubbingwaste converter gas.

BACKGROUND OF THE INVENTION

In the scrubbing of converter gases containing calcium-oxide particlesand carbon dioxide with water it is a standard practice to recirculatethe scrubbing water and then to settle out the particles, decanting andrecycling the liquid. Fresh water is added to the wash water to make uplosses.

A particular problem with such arrangements is that dust particles andcalcium deposit in the water-recirculating system. These deposits are aresult of the fact that the calcium-oxide particles picked up by thewash water form soluble calcium hydroxide which dissolves. The calciumion combines with the carbon dioxide in the water to form insolublecalcium carbonate (CaCO₃).

It has been attempted to avoid these deposits by adding products to thewater which ensure the precipitation of the dissolved calcium in theform of insoluble calcium compounds prior to recirculation. Theadditives necessary to accomplish this are relatively expensive and mustbe used in relatively large quantities, i.e., at leaststoichiometrically equivalent to the calcium content of the gas treated.Approximately 75% of the wash water must be replaced with fresh water inconventional systems. Obviously such a system leads to furtherdifficulties in the disposal of the contaminated wash water and thesupply of the necessary large quantities of fresh water.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide animproved method of scrubbing particle-containing (e.g. metallurgicalconverter) gases.

Another object is the provision of an improved system for cleaning wasteconverter or stack gases.

Yet another object is to provide such a system wherein the formation ofdeposits in the wash-water circulating system is avoided.

SUMMARY OF THE INVENTION

The invention resides in scrubbing a gas containing calcium-oxideparticles with an aqueous solution of the carbonate ion (CO₃ ⁼) or thehydrogen carbonate ion (HCO₃ ⁻) prior to any substantial solubilizationof the calcium oxide to form an insoluble calcium-carbonate sheath onthe particles, collecting the calcium-carbonate-coated particles ofcalcium oxide in the aqueous solution, settling the coated particlesfrom the solution and recirculating it to scrub additional quantities ofthe gas after replenishing the carbonate or hydrogen carbonate ioncontent.

Where the gas is a converter waste gas, e.g. derived from a Bessemer,Thomas or other converter, it generally will contain sufficient levelsof CO₂ so that at least part of the carbonate or hydrogen carbonate ionlevel can be maintained by solubilization of this gas component. To thisend there is maintained a CO₂ -solubilizing quantity of an alkali metal(Na, K, Li) in the recirculated solution. Most generally, however, smalladditions of these alkali-metal carbonates or hydrogen carbonates willbe required.

According to the present invention alkali salts are used in the form ofcarbonates and/or hydrogen carbonates or alkali-hydroxides as additivesto the wash-water in a system for removing particles, in particularcalcium-oxide particles, from gases containing carbon dioxide. Afterseparation of the solid particles from the water by means ofsedimentation and decantation and after the addition of fresh water tothe wash water in order to replace that quantity lost, the wash waterand the additive are recycled so as to inhibit the dissolving of thecalcium from the calcium-oxide particles.

According to a particular feature of this invention soda (Na₂ CO₃) isused as the additive.

The invention is based on the fact that the calcium oxide on the surfaceof the calcium-oxide particles becomes calcium hydroxide relativelyslowly and that the thus formed calcium hydroxide, before it isdissolved in the water, is transformed by the carbon dioxide dissolvedin the water into calcium carbonate. In this manner there is formed onthe calcium-oxide particles a virtually inactive coating whichcompletely eliminates any further reaction between the calcium oxide andthe water or the calcium hydroxide with the carbon dioxide. Dissolvedcalcium thus is no longer present in the recycled wash water, as theinactivated calcium-oxide particles are readily and easily moved by thesettling and decantation step. The lye or hydroxide ions formed by thereaction of the carbonate with the calcium hydroxide on the surface ofthe calcium-oxide particles react with the carbon dioxide in the wastegas to form soluble carbonates or hydrogen carbonates (bicarbonates).Thus it is necessary to dose the circulating wash water with a quantityof carbonate which is equal to the difference between the elutriatedcarbonate quantity and the newly formed carbonate quantity. Thus theadditive according to the present invention is self-regenerating.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages will become morereadily apparent from the following description reference being made tothe accompanying drawing in which:

FIG. 1 is a largely schematic view illustrating the system according tothe present invention; and

FIG. 2 is a large-scale view of a calcium-oxide particle illustratingthe principles of the present invention.

SPECIFIC DESCRIPTION

As shown in FIG. 1 a converter gas containing calcium-oxide particlesand carbon dioxide is passed upwardly through a scrubbing tower 1 asdescribed in commonly assigned patent application Ser. No. 269,773 filedJuly 7, 1973 by G. HAUSBERG et al (now U.S. Pat. No. 3,906,078).

Aqueous sodium carbonate is used as the scrubbing liquid so as to reactwith the calcium oxide particles and the carbon dioxide in the convertergas to form a solution and suspension that is settled, the liquid beingdecanted, at 2. As is shown in FIG. 2 the calcium oxide on the surfaceof a calcium-oxide particle 3 is converted relatively slowly intocalcium hydroxide and which, before it is dissolved in the water, reactswith the carbonate dissolved in the water to form calcium carbonate.Thus a coating 4 of calcium carbonate is formed on each of the particles3 so as to prohibit further reaction of the calcium oxide with the wateror of calcium hydroxide with the carbon dioxide. Thus calcium ionsthemselves are not present in the wash water and the inactivecalcium-oxide particles are readily removed at the elutriation stepshown at 2.

The system is dosed at 5 with a quantity of carbonate equal to thedifference between the amount of carbonate lost at the elutriation stepand the newly formed carbonate. Caustic soda (NaOH) is added at 6instead of sodium carbonate to treat the gas with a very high carbondioxide content.

The necessary carbonate concentration in the wash water depends on theamount of time the water contacts the gas in the scrubbing tower 1, onthe carbon-dioxide content of the gas, and on the quantity and particlesize of the calcium-oxide particles in the gas. In accordance with thepresent invention between 0.2 and 2 g/l, preferably between 0.4 and 1g/l of carbonate is used in the wash water. It is also within the scopeof this invention to use alkali hydroxide rather than an alkalicarbonate according to the carbon-dioxide concentration in the gas. Suchan alkali hydroxide in one pass through the washer 1 reacts with thecarbon dioxide to form carbonates.

The method according to the present invention can be used with theaddition of very little fresh water so that it is indeed a closedsystem. In addition a relatively small quantity of the additives is alsonecessary so that the entire operation is relatively inexpensive.

SPECIFIC EXAMPLE

When the system according to the present invention is used with theprior-art method and an overall water volume of 5500 m³ and athroughflow in the washer 1 of 1040 m³ /h, it is necessary to add to thecirculating system 800 m³ /h, of fresh water so as to replace 80 m³ /hwater lost through evaporation and 720 m³ /h which is drawn off at theseparation step to prevent the forming of deposits in the system.

Using the same overall water volume and the same throughflow in thewasher 1, but adding to the fresh water in the system 5500 kg of soda,thus at a concentration of 1 kg/m³, it was possible to conduct theoperation with loss of only 80 m³ /h of the water at the elutriationstep. This, added to the inevitable evaporation of 80 m³ /h means thatonly 160 m³ /h fresh water was necessary. In addition to this it wasnecessary to add 105 kg/h of the soda to the system, in which case therewere absolutely no deposits in the system.

Thus in accordance with the present invention it is possible to save 640m³ /h of fresh water by using 105 kg/h of soda. Since one kilogram ofsoda costs approximately as much as 1 m³ of fresh water the systemaccording to the present invention is clearly much less expensive to runthan the prior-art system.

I claim:
 1. A method of treating a gas containing calcium-oxideparticles, said method comprising the steps of:scrubbing said gas with aliquid consisting essentially of an aqueous solution of alkali-metalcarbonate or alkali-metal hydrogen carbonate to form an insolublecarbonate sheath on said particles prior to the solubilization ofcalcium ion therefrom and entraining the resulting calcium-carbonatecoated particles with the solution; separating the entrained particlesfrom said solution by settling and decantation of the solution;thereafter adding water to said solution and replenishing the carbonateor hydrogen carbonate concentration thereof to form a regeneratedsolution; and thereafter scrubbing additional quantities of said gaswith said regenerated solution.
 2. The method defined in claim 1 whereinsaid gas is a converter gas containing carbon dioxide in addition tosaid particles, and said solution is regenerated by adding an alkalimetal carbonate, hydrogen carbonate or hydroxide to the solution.
 3. Themethod defined in claim 2 wherein soda is used as said carbonate in aconcentration between 0.2 g/l and 2 g/l.
 4. The method defined in claim3 wherein said soda is employed in a concentration of between 0.4 g/land 1 g/l.